1. Field of the Invention
This invention relates to a control device which drives controllably a recording medium such as recording paper and photosensitive drum.
2. Description of the Prior Art
As a conventionally known type of the drive-control device for a recording medium, explanation will be given hereinbelow in reference to FIGS. 1a, 1b and 1c and 2 of the accompanying drawing taking a laser beam printer as an example, wherein a laser beam modulated by recording signals scans the surface of a photosensitive drum rotating at a constant rotational speed to thereby record informations on the photosensitive drum in the form of a latent image, and then this latent image is developed with a toner, followed by transfer of the developed image onto the recording sheet.
That is to say, FIG. 1(a) illustrates changes in the rotational speed of the photosensitive drum, wherein the abscissa denotes a time t and the ordinate represents a rotational speed .nu. with .nu..sub.0 as a preset rotational speed. Now assume that, at a time instant t.sub.0, the rotational speed slows down to .nu..sub.1 due to abrupt change in load applied thereto. Then, at time t.sub.1, a control circuit starts its control action and imparts to a motor for driving the photosensitive drum a drive pulse Pa as shown in FIG. 1(b) in accordance with a width in variation of the rotational speed (.nu..sub.0 -.nu..sub.1). However, when an excessive pulse Pa is imparted to the motor, there takes place a hunting phenomenon, by which the rotational speed exceeds the preset speed .nu..sub.0 at the time instant t.sub.2 and reaches a speed .nu..sub.2. At this instant, the current supply to the motor is interrupted for an instant until the drum reduces its rotational speed. On the contrary, however, when the rotational speed becomes too slow, another pulse Pb as shown in FIG. 1(b) is imparted to the motor. By repeating such pulse imparting actions, the conventional control device managed to bring the rotational speed of the drum closer to its ideal value .nu..sub.0.
FIG. 1(c) shows irregularities in density of an image produced by the repetitive pulse imparting and non-imparting operations. As will be seen from the illustration, since it is due to density variations in the auxiliary scanning direction caused by speed variations in the auxiliary scanning direction that the irregularities in the drum rotation results in the image density irregularities, the density in image varies in a recurring pattern of "dense, sparse, dense, sparse, . . . " due to the hunting phenomenon as shown in FIG. 1(c) with the result that such recurring "dense-sparse" pattern is seen as the irregularities in the image density. With such sparse portions in the produced image, the density variations become more conspicuous upon comparing them with the dense portions.